1 CHAPTER 24 THE ORIGIN OF SPECIES What is a species Species is a group whose members possess similar anatomical characteristics and have the ability to interbreed Many species were originally distinguished by morphology Two organisms that look alike must be the same species However biological species are based on the ability to reproduce Individuals may look alike but not reproduce and therefore are not the same species Individuals may look different but can reproduce and therefore are the same species See fig 24 2 OR The species category can be conceptualized in many ways 1 Biological Species Concept a population or group of populations whose members have the potential to interbreed in nature and produce viable fertile offspring but are not able to produce viable fertile offspring with members of other populations 2 Morphological Species Concept measurable anatomical criteria 3 Ecological Species Concept in terms of ecological niche the sum of how members of the species interact with the nonliving and living parts of their environment 4 Phylogenetic Species Concept as the smallest group of individuals that share a common ancestor forming one branch on a tree of life Biological Species are based on the ability to reproduce The organisms can produce viable fertile offspring but they cannot produce viable fertile offspring with other groups of organisms What about organisms which reproduce asexually or where the reproduction hasn t been observed Morphological Species defines a species by measurable anatomical criteria Many species were originally distinguished by morphology If two organisms look alike they must be the same species As the DNA for more organisms is studied the placement of organisms within the taxonomic tree becomes based more on the DNA However biological species are based on the ability to reproduce Individuals may look similar but not reproduce and therefore are not the same species See fig 24 2 Or Individuals may look different but can reproduce and therefore are the same species Ex Different breeds of dogs Ecological Species defines a species in terms of ecological roles niches based on behavior Phylogenetic Species defines a species as the smallest group of individuals that shares a common ancestor sharing an evolutionary history But how do we know the evolutionary history The ability to examine small but meaningful differences between organisms has resulted in changes in the way organisms are classified 2 Fossils indicate the previous existence of many species Most now extinct Many have no living descendants Evolution is the force behind speciation Speciation development of a new species where there was one before Two main components 1 Isolation of the population 2 Genetic diversity Reproductive barriers separate species see fig 24 3 Reproductive barriers impede mating between species or prevent fertilization if members of different species attempt to mate Reproductive Isolation is a prerequisite for the formation of species A barrier prevents two species from producing fertile hybrids Most species have more than one barrier Prezygotic isolating mechanisms hinders zygote formation 1 Geographical isolation Prevents populations from interbreeding but if the species are put together 2 Ecological Habitat Isolation Isolation because of where they choose to live tree top versus forest they may mate floor 3 Temporal Isolation breeding seasons occur at different times 4 Behavioral Isolation courtship rituals are unique to a certain species 5 Mechanical incompatibility physiological barrier Great Dane and Chihuahua 6 Gametic Incompatibility or isolation sperm or one species cannot fertilize egg of another species Postzygotic isolating mechanism hinders zygote development and propagation 1 Hybrid Viability genes of the different parent species may interact and impair the hybrid s development can t build a proper nest 2 Hybrid Fertility hybrid is sterile or has greatly diminish fertility 3 Hybrid Breakdown some first generation hybrids are fertile but when they mate with another species or with either parent species offspring of the next generation are feeble or sterile The mechanisms of speciation 1 Allopatric speciation see fig 24 6 The gene pools must be geographically isolated 2 Sympatric speciation The population shares the same geographic area Geographical isolation can lead to the origin of species Allopatric speciation Allopatric speciation the formation of new species in populations that are geographically isolated from on each other 3 The type of necessary barrier depends on the species A new species can originate without geographic separation Sympatric speciation Sympatric speciation the formation of a new species in populations that live in the same geographic area Usually result from radical mutations Polyploidy an extra chromosome set s it is a result of an accidental cell division Autopolyploidy individual has more than two chromosome sets all derived from a single species Allopolyploid a fertile individual that has more than two chromosomes sets as a result of two different species interbreeding and combining their chromosomes common among plants Offspring survive just fine or even better but can t reproduce No sperm or eggs Hybrid Zones provide insight to factors that cause reproduction isolation Hybrid Zones regions where two related populations that diverged after becoming geographically isolated makes secondary contact and interbred where their geographic ranges overlap Gradualism versus Punctuated Equilibrium Gradualism is the traditional view of evolution New species evolve from gradual accumulation of characteristics However paleontologists frequently do not find transitional forms species appear and disappear suddenly Punctuated Equilibrium speciation occurs during brief periods of rapid change followed by a period of little change Rarity of transitional fossils Punctuated equilibrium
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